DESCRIPTION: (Adapted from the application) This research is to improve quantification of radiation absorbed dose to tumors in patients following administration of therapeutic amounts of radiopharmaceuticals using single-photon emission computed tomography (SPECT). The macro-dosimetric result is to be correlated with local response to therapy and a tracer-imaging protocol will be developed. Non-Hodgkins' B-cell lymphoma patients are being successfully treated by I-131 labeled monoclonal antibodies. A dose-monitoring protocol has improved on standard procedure and is producing more plausible dose-response curves. The investigators will increase the data-processing efficiency in order to apply it to more patients. They will also further improve SPECT quantitative accuracy as follows: a) To determine when energy extrapolation of x-ray computed tomography (CT) values for SPECT attenuation correction needs to be replaced by radionuclide transmission scanning, they will compare the two methods for an anthropomorphic phantom. b) To extend accurate correction for Compton scattering they will characterize by Monte-Carlo simulation where the dual-energy-window method begins to fail. They will employ spectral fitting, using a special computer to acquire local energy spectra. c) To obtain accurate results from CT regions of interest (ROIs) superimposed on SPECT slices, they will develop a region-growing computer program that accounts for poorer SPECT resolution and slight misalignments. They will also improve accuracy of the dosimetry by investigating (1) the intra-observer variability of the CT tumor volume, (2) the shape of the conjugate-view time-activity curve (CV TAC), and (3) the behavior of % uptake as administered activity is increased. To develop dosimetric estimation from SPECT tracer studies, they will employ constrained, least-squares fitting of the projection data to compensate for poor counting statistics. Finally, to gain fuller information from human-tumor studies in animals, a miniature, implantable beta spectrometer will be developed, calibrated, and tested.